Scaling and Crusting
Definition
Scale is a visible accumulation of fragments of the horny layer of the skin (stratum corneum); it represents the final product of epidermal keratinization.7 The process of forming the stratum corneum is termed cornification.
Histologically, scale is recognized as hyperkeratosis and can be subdivided into parakeratosis (cornification with nuclear retention) or orthokeratosis (cornification without nuclear retention). In some conditions parakeratosis and orthokeratosis may be present together. Grossly the scale varies in appearance. The color may be white, silver, yellow, brown, or gray. The consistency may be like that of bran flakes or powdery, coarse, greasy, or dry. Scale can be either loose or adherent to the skin or hair shafts.Crusts are composed of variable amounts of serum; cells (leukocytes, erythrocytes, keratinocytes); fibrin; infectious agents (bacteria and fungi); debris; and/or medications. They often cover erosions or ulcerations. Heaped-up crusts are referred to as vegetations. On the basis of their histologic composition, crusts may be subdivided into cellular, serocellular, serous, and hemorrhagic types.
Mechanisms of Scale and Crust Formation
Scaling results from increased desquamation (exfoliation) of the stratum corneum. Exfoliation is the final stage of keratiniza- tion, the process by which the permanent population of cells of the basal layer of the epidermis divides, undergoes specific patterns of differentiation, and progresses toward the surface, where it is shed.7,9 Excessive eXfoliation and scale formation occur when the rate of keratinization is accelerated; when trauma to the surface of the epidermis (chemical, mechanical, or thermal) loosens the stratum corneum; or when the structures sustaining epidermal intercellular cohesion (such as transmembrane proteins) are destroyed, resulting in a loss of cohesion between epidermal cells.
Because crusts are composed primarily of serum and cells, their presence on the skin surface implies that vascular and epidermal permeability has increased to permit their formation. Serum and inflammatory cells are released into the tissues from the dermal vasculature and then cross the epidermis to the skin surface either through erosions or ulcerations or by permeating between the intercellular spaces. The exudate dries on the skin surface, in combination with any medication or debris that was already present on the hair or skin, to form the visible crust. Desquamating keratinocytes may be swept up in the exudate and become part of the crusts. Bacteria frequently invade crusts after they have formed and will be noted on histologic examination, even though they may not be a factor in the pathogenesis. Fungal organisms, when present, are more likely to be important to the pathogenesis of the underlying disease process.
Approach to Diagnosis of Scaling and Crusting
The most important factor in determining the underlying cause of scale or crust formation in either a horse or a ruminant is to determine if the patient is pruritic and if some or all of the lesions are induced by self-trauma. If pruritus is a feature, the approach to diagnosis of pruritus should be used because all pruritic diseases can cause scale and crust formation. If the patient is not pruritic, the most important differential diagnoses include infectious diseases (particularly dermatophilosis and dermatophytosis), nutritional disorders, toxicities, autoimmune disease (pemphigus foliaceus), cutaneous filariasis, photosensitization, irritant contact reactions or burns, and diseases of uncertain cause (e.g., in equine patients, sarcoidosis, aural plaques, and primary seborrhea) (Box 11.5).7,10 Viral diseases are important nonpruritic causes of scaling and crusting in ruminants, although they are usually associated with ulceration and erosion with involvement of the oral cavity and mucocutaneous regions.
Rarely, congenital genetic diseases such as ichthyosis may cause excessive crusts and scale, as has been reported in cattle and sheep.The following steps are a guide to the diagnosis of scaling and crusting in horses and ruminants:
1. History (see Fig. 11.1)
a. In particular, determine if the patient is pruritic.
b. Determine if contact animals of the same and/or different species are affected. If they are, a contagious problem
■ BOX 11.5
Most Common Causes of Scaling and Crusting in Horses and Ruminants
Ectoparasites
Sarcoptes scabiei (ruminants)
Psoroptes cuniculi (goats)
Psoroptes ovis (sheep)
Lice
Chorioptes species (horses and cattle)
Infectious
Dermatophilus congolensis
Staphylococcal pyoderma
Dermatophytosis
Immune-Mediated
Pemphigus foliaceus (horses and goats) Photosensitization (horses)
Nutritional
Zinc deficiency (ruminants)
should be considered: fungal (dermatophytosis), bacterial (dermatophilosis), viral, or parasitic.
2. Physical examination (see Fig. 11.2)
a. Look for evidence of excoriation, suggesting that pruritus is a feature.
b. Inspect the coat closely for small but grossly visible parasites such as lice or their eggs.
c. Inspect contact animals for evidence of disease, suggesting a contagious cause.
3. Skin scrapings
4. Acetate tape preparations
5. Dermatophyte culture and KOH preparation
6. Dermatophilus preparation
7. Biopsy for routine histopathologic examination (careful histologic examination of scale and crusts is essential to the search for the underlying cause of their formation)
8. Microfilarial preparation
9. Biopsy for direct immunofluorescence testing
10. Bacterial culture and sensitivity